This document describes an air pulse fruit harvester that uses cyclic pulses of air to remove olives from trees, providing a non-contact alternative to mechanical harvesters. It works by exciting the natural resonance frequency of the olive stems to cause fatigue and dropping. The harvester uses a spinning rotor to divert a steady air stream into pulsating jets that target olives as trees are swept. Prototyping showed pulses can be controlled by fan speed, rotor speed, and number of openings to target the optimal resonance range without jet interference. The non-contact design aims to improve efficiency and reduce plant damage over mechanical methods.

1. Air pulse fruit harvester
Universidad Nacional del Litoral
CONICET
Angers SRL

2. Air pulse fruit harvester
The advent of intensive
farming methods for
olive requires of new
mechanical methods to
replace manual crop
harvest, due to its
inefficiency and
inherent risks.

3.  The vast majority of current designs for
automatic harvesting are mechanical.
The fruit drop occurs shaking the
ground by rotating counterweights
applied to the trunk or hitting the
branches with rods.
 This type of harvester produces damage
to the leaves, fruit buds and in general
the whole structure of the plant.
 They also tend to be heavy, damaging
the roots especially in areas of low soil
compaction.

4.  Harvesting methods that do not have physical
contact with the ground are sought long ago.
 One possibility is the use of strong air streams.
In fact, it is the natural way the fruits usually fall
during storms and high winds.

5. However, the wind speed required to produce fruit
drop is too high, requiring very powerful turbines
with the disadvantage of high weight and plant
damage (as with mechanical harvesters).
It is estimated that the force
required to remove an olive
is 300 grf (grams force),
while the drag force
produced by a wind of
360kmh is less than 50grf.

6. The force required to pick the
olive can be dramatically reduced
cyclic loads are applied, causing
material fatigue.
Steel spring broken by cyclic
fatigue

7. All mechanical systems have "resonance
frequencies". By applying cyclic loading at these
frequencies it is more likely that the system
breaks.
The Tacoma Narrows bridge was destroyed
by the resonance produced by a wind of
67kmh whose aerodynamic loads excited
resonant mode at 0.2Hz.
The Angers bridge was destroyed in 1850
when an army of 478 soldiers marching on it
excited resonance and caused its collapse.
Which is the best frequency for
olives?

8. 
Here we can see the most important
resonance modes
First mode (lowest frequency)
Second nd mode (highest
frequency). Note the
compression stresses (red) and
traction (blue) are higher.

9. 
It has a low frequency and
produces low stress on the
stalk.

The base of the peduncle and
the fruit itself move in phase.

There is a large displacement
of the center of mass of the the
fruit (hence low frequencies)
(video elastld-w1.asf)
First resonance mode

10. 
Typically has higher
frequency and produce larger
stress on the peduncle.

The base of the peduncle and
the fruit move in opposite
phase.

Small displacements of the
center of gravity of the olive.
Practically the fruit rotates
around its center of gravity
(vide elastld-w2.asf)
Second resonance mode

11. How can we produce cyclic loads?

There are
several
conceptual
designs of
pulsed air
harvesters.

Most of them
generate
pulses by
cyclically
opening and
closing an air
passage.

12. But accelerating and decelerating
the air flow is inefficient!!
This strategy is very inefficient because the
flow accelerates and decelerates, which
requires large amounts of energy due to the
inertia of the fluid . While air is a lightweight
fluid the high speeds required (150kmh) and
pulse rates required (approx 20Hz) imply an
important effect of fluid inertia. To get an
idea, the thrust produced by the air jet in this
conditions is 240 Kgf.

13. How can we produce pulsating air
without stopping the flow?

The solution is to DIVERT the flow without
STOPPING it!
Analogy: Aikido
is a martial art that
is based on divert
and redirect the
momentum of the
opponent instead
of opposing it
directly.

14. Alternative vs. rotative movement
Finally, there is a further improvement. Instead of
moving alternatively the device or part of it,
diverting the flow is produced by a rotor.
This has many advantages:

Simplifies the mechanical design

Avoids cyclic loading on the structure
(fatigue)

Further improves aerodynamic efficiency
(steady flow in a rotating system)

15. How can we divert the air stream?
Let's first see
what happens
when the rotor
is stationary.
The rotor blades
divert the the
main stream in
three powerful
jets.

16. One way to visualize the flow is
think as if the air were composed
of tennis balls. To begin, we see
here how the air behaves when
the rotor spins at low speeds.
(video shaker42.asf)
How can we divert the air stream?

17. Spinning rotor
As the rotor starts spinning the
points where the jets are
emitted perform a circular
motion and the air jets are bent
into helices. (videos
shaker41.asf)

18. But the air stream follows a straight
line!!
In the figures and animations
presented tennis balls
(representing the air blown by
the fan) form a helix-shaped
jet. This would seem to suggest
that the air jet is bent but it's
really not! Each ball follows a
straight path. It is the change
in position of the point of
emission that generates the
helical pattern. (see video
shaker40.asf). This is in line
with the strategy of disturbing
the flow as little as possible so
that the aerodynamic
efficiency is improved.

19. How the cyclic load on the fruit is
produced?
As the air jets spin,
the fruit passes
alternately through
the jet zone and
outside the jet zone
producing the
desired cyclic load
(see video shaker39.asf)

20. How the pulse generator is applied
to the whole tree?
By moving the
generator sweeps
a fringe in the
tree line.
Depending on the
configuration this
area may have a
height of 1.5m
and a depth of
2m.(see videos
shaker36.asf-shaker37.asf-
shaker38.asf) Área barrida por un generador

21. How the pulse generator is applied
to the whole tree?
Using 4 pulse
generators, two
on each side of
the row and
covering the
top and bottom
part of the tree.
(see videos
shaker33.asf-
shaker34.asf-
shaker35.asf)
Area barrida por 4 generadores

22. Novelty of the design

A harvesting technology that does not make physical
contact with the tree. The picking is produced by pulses of
air, producing fatigue in the peduncle.

High aerodynamic efficiency: The whole design is based
on diverting the airflow, never stopping it.

The mechanical design is based on a rotor and has no
reciprocating components.

The air stream follows a straight path.

23. Efficiency of the air pulse

The ideal pulse should have an
intensity as high as possible
and a frequency as close as
possible to the frequency of
resonance.

The intensity is measured as
the difference between peak
and valley pressure.

The generator must produce a
pulse at the resonance
frequency, but due to
variations in the weight and
stiffness, a frequency range is
covered. The generator must
be able to change the
frequency, keeping an
appropriate intensity
throughout this range.

24. Controls parameters

Fan power: Directly affects the pulse intensity.

Speed and number of openings of the rotor: directly influence the
frequency.
However, increasing too
much the rotational speed or
the number of openings may
indirectly affect adversely
the amplitude, as the air jets
begin to interfere with each
other.
The jets interfere
with each other
because the
frequency is very
high.
The jets interfere
with each other
because there are
many openings.

25. When the jets interfere with each
other?
That can be determined either by
Laboratory
experimentation
Computational Fluid Dynamics
simulation

26. Pulse generator prototype

A full scale pulse generator prototype was built.

The fan rotates up to 2200rpm, driven by a hydraulic motor which
in turn is driven by a 40 HP electric motor.

The rotor is controlled by an electromagnetic brake.

Pulse intensity measurements based on different parameters
measuring equipment based on Pitot tubes and a computer data
acquisition were performed.